A gas sensor which measures the concentration of oxygen holds a cover which covers a portion of a gas sensing device between the gas sensing device or a device body into which the gas sensing device is inserted and an inner peripheral hole of a housing. The exhaust gas is admitted to a device portion of the gas sensing device through holes of the cover. The device portion measures the concentration of oxygen contained in the exhaust gas.
For instance, Japanese Patent First Publication No. 2013-117521 discloses a gas sensor having a device body which is of a cylindrical shape with a bottom and equipped with a flange extending outwardly in a radial direction. The device body is inserted into a tubular metal shell which has a seating surface facing the flange. An annular metallic portion is disposed between the flange of the device body and the seating surface of the metal shell to avoid a positional shift of the metallic portion in the radial direction. The metal shell corresponds to a housing. The metallic portion corresponds to a cover.